The active center of iron proteins is studied by 57Fe Mossbauer spectroscopy and magnetic resonance. Our aim is to establish correlations between spectroscopic and structural features and to elucidate the physical basis of the functions performed by the proteins. The ESR-silent states of the iron, Fe24 and Fe4 ion, are of particular interest since low-temperature, high-field Mossbauer spectroscopy can provide more information than any other method about the electronic groundstate of the iron in these functionally important reaction intermediates. Specifically, the following systems are studied; Cytochrome P450cam from the camphor hydroxylase of Pseudomonas putida, a monoxygenase that closely resembles mammalian cytochromes of the P450 type. Compounds I and II of horseradish peroxidase and chloroperoxidase which are claimed to have the heme iron in the Fe(IV) state. Photosynthetic reaction centers in the native and reduced state. BIBLIOGRAPHIC REFERENCES: M. Sharrock, P.G. Debrunner, C. Schulz, J.D. Lipscomb, V. Marshall and I.C. Gunsalus. Cytochrome P450cam and its complexes - Mossbauer Parameters of the Heme Iron. Biochem. Biophys. Acta 420, 8-26, 1976. P.G. Debrunner, Enzyme Systems. In "Applications of Mossbauer Spectroscopy", R.L. Cohen, Ed., Vol. I. 171-196, Academic Press, New York, 1976.